Field electron emission source having carbon nanotubes and method for manufacturing the same

ABSTRACT

An exemplary method for manufacturing a field electron emission source includes: providing a substrate ( 102 ); depositing a cathode layer ( 104 ) on a surface of the substrate; providing a carbon nanotube paste, coating the carbon nanotube paste on the cathode layer; calcining the carbon nanotube paste to form a carbon nanotube composite layer ( 110 ); and, irradiating the carbon nanotube composite layer with a laser beam of a certain power density, thereby achieving a field electron emission source.

BACKGROUND

1. Field of the Invention

The present invention relates to field electron emission sources havingcarbon nanotubes and methods for manufacturing the same.

2. Discussion of Related Art

Field emission displays (FEDs) are a relatively new and rapidlydeveloping flat panel display technology. Compared to conventionaltechnologies, e.g., cathode-ray tube (CRT) and liquid crystal display(LCD) technologies, field emission displays are superior in having awider viewing angle, lower energy consumption, a smaller size, and ahigher quality display. A field electron emission source is an essentialcomponent in FEDs and has been widely investigated in recent years.

Carbon nanotubes (CNTs) are very small tube-shaped structures,essentially having a composition of a graphite sheet rolled into a tube.CNTs produced by arc discharge between graphite rods were discovered andreported in an article by Sumio Iijima entitled “Helical Microtubules ofGraphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). CNTs haveextremely high electrical conductivity, very small diameters (much lessthan 100 nanometers), large aspect ratios (i.e. length/diameterratiosgreater than 1000), and a tip-surface area near the theoreticallimit (the smaller the tip-surface area, the more concentrated theelectric field and the greater the field enhancement factor). Thus, CNTscan transmit an extremely high electrical current and have a very lowturn-on electric field (approximately 2 volts/micron) for emittingelectrons. In summary, CNTs are among the most favorable candidates forelectron emission terminals of a field electron emission source, and canplay an important role in FED applications.

A conventional method for manufacturing the field electron emissionsource utilizes a screen-printing process. In this method, a CNT pastehaving CNTs and conductive paste is formed on a cathode and thencalcined to form a CNT composite layer. Most CNTs embedded in the CNTcomposite layer cannot emit electrons. For this reason, a surface of theCNT composite layer is cut and polished to form electron emissionportions. However, in this mechanical method, the formation of theelectron emission portions cannot be accurately controlled. Further, thefield electron emission source has a low field electron emissionefficiency due to a shielding effect caused by closer, adjacent CNTs.

Therefore an accurately controlled method for manufacturing fieldelectron emission sources and a field electron emission source with highfield electron emission efficiency are desired to overcome theabove-described problems.

SUMMARY

A method for manufacturing a field electron emission source includes:providing a substrate and depositing a cathode layer on a surface of thesubstrate; providing a carbon nanotube paste and coating the carbonnanotube paste on the cathode layer; calcining the carbon nanotube pasteto form a carbon nanotube composite layer; and, irradiating the carbonnanotube composite layer with a laser beam of a certain power density,thereby achieving a field electron emission source.

The present method for manufacturing the field electron emission sourcecan have the following advantages over conventional methods. First, themethod can be performed rapidly and easily due to a high energy densityof the laser beam. Secondly, the field electron emission source has ahigh resolution because the laser beam creates a sharp edge on theelectron emission portion. Thirdly, the electron emission portions ofthe field electron emission source can be accurately selected bycontrolling the movement of the laser beam. Lastly, the field electronemission source has high field emission efficiency due to protrudingCNTs in the electron emission portion.

Other advantages and novel features of the present method and a relatedfield electron emission source will become more apparent from thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present method for manufacturing a field electronemission source and of the present field electron emission source may bebest understood with reference to the following drawings. The componentsin the drawings are not necessarily drawn to scale. Instead, theemphasis is placed upon clearly illustrating the principles of thepresent method and field electron emission source.

FIG. 1 is a flow process chart, showing a method for manufacturing afield electron emission source according to one embodiment.

FIG. 2 is a schematic, cross-sectional view of a field electron emissionsource according to one embodiment.

FIG. 3 is a Scanning Electron Microscope (SEM) image, showing a CNTcomposite layer of the field electron emission source of FIG. 2.

FIG. 4 is an SEM image, showing a protrusion of a CNT composite layer ofthe field electron emission source of FIG. 2.

FIG. 5 is a photo showing the field electron emission source in aworking state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference will now be made to the drawings to describe preferred andexemplary embodiments of the present invention in detail.

Referring to FIG. 1, a method for manufacturing a field electronemission source includes the steps of:

(a) providing a substrate, and depositing a cathode layer on a surfaceof the substrate;(b) providing a carbon nanotube (CNT) paste and coating the CNT paste onthe cathode layer;(c) calcining the CNT paste to form a CNT composite layer; and(d) irradiating the CNT composite layer with a laser beam of a certainpower density, thereby achieving a field electron emission source.

In step (a), a pattern of the cathode layer is deposited in apredetermined region on a surface of the substrate by a conventionalmethod, such as the sputtering method. The substrate can be made of anysuitable material, e.g., glass, plastic, or metal. The cathode layer ismade of one or more conductive metal materials, e.g., gold, silver,copper, or any one of their alloys.

In step (b), the CNT paste is prepared by mixing CNTs in a knownconductive paste, such as a silver paste. CNTs account for about 5%-15%of the total mass of CNT paste. CNTs can be obtained by a conventionalmethod, such as chemical vapor deposition, arc discharging, or laserablation. The lengths of the CNTs range from about 5 microns (μm) toabout 15 μm. The CNT paste can be coated on the cathode layer using ascreen-printing method.

In step (c), solvent and volatile components of the CNT paste are firstvolatilized. Then, the resultant paste is calcined in air or in vacuumat about 1⁻¹⁰ torr, for a period of about 15 to 60 minutes. Thereafter,the CNT paste is transformed into a CNT composite layer on the cathodelayer, and the CNT composite layer becomes firmly attached to thecathode layer. In the CNT composite layer, CNTs are uniformly embeddedand rarely exposed on the surface.

In step (d), the high power density laser beam irradiates a selectiveportion of the surface of the CNT composite layer, thereby increasingthe temperature of the selected portion rapidly. The portion of the CNTcomposite layer expands and forms a protrusion (i.e., both CNTs and theresultant paste protrude). Next, the resultant paste of the CNTcomposite layer is removed by a laser beam to expose CNTs in theprotrusion which function as electron-emitting terminals when a currentflows through. As a result, the shielding effect of the adjacent CNTs isreduced, and accordingly, the field emission efficiency of the CNTs isimproved. The power density of the laser beam is about 10⁴-10⁵ V/mm²(volts per square millimeter), ideally, around 7×10⁴ V/mm². If the powerdensity of the laser beam is insufficient, a groove is formed in the CNTcomposite layer, and CNTs thereby become exposed in the groove withterminals of the CNTs being lower than the CNT composite layer. In suchcase, the shielding effect of adjacent CNTs and the like are increased,and the CNTs cannot emit electrons efficiently. If the power density ofthe laser beam is excessive, CNTs fuse. The laser beam can be movedalong a predetermined route forming a pattern of the exposed CNTs in acorresponding region on the surface of the CNT composite layer. Themoving rate of the laser beam should be approximately 800 mm/s(millimeters per second) to 1500 mm/s, ideally, around 1000 mm/s. Theroute of the laser beam can be accurately controlled by a computer.

Referring to FIG. 2, a field electron emission source 100 manufacturedby the method in FIG. 1 is shown. The field electron emission source 100includes a substrate 102, a cathode layer 104 deposited on the substrate102, and a CNT composite layer 110 coated on the cathode layer 104. TheCNT composite layer 110 includes a resultant paste 112 and CNTs 114. Onepart of the CNTs 114 is embedded in the resultant paste 112, and theother part of the CNTs 114 is exposed and protruded from the resultantpaste 112. The protruded CNTs are higher than the CNT composite layer110 by 8-12 microns.

Referring to FIGS. 3 and 4, a scanning electron microscope (SEM) imageof the field electron emission source and an amplified SEM image of theprotruded CNTs are shown, respectively. Referring to FIG. 5, the fieldelectron emission source is shown in a working state.

It is believed that the present embodiments and their advantages will beunderstood from the foregoing description, and it will be apparent thatvarious changes may be made thereto without departing from the spirit orscope of the invention or sacrificing all of its material advantages,the examples hereinbefore described merely being preferred or exemplaryembodiments of the invention.

1. A method for manufacturing a field electron emission source,comprising: providing a substrate, and depositing a cathode layer on asurface of the substrate; providing a carbon nanotube paste, and coatingthe carbon nanotube paste on the cathode layer; calcining the carbonnanotube paste to form a carbon nanotube composite layer; andirradiating the carbon nanotube composite layer with a laser beam, andthereby achieving a field electron emission source.
 2. The method ofclaim 1, wherein the cathode layer is deposited on the substrate by asputtering method.
 3. The method of claim 1, wherein the substrate ismade of a material selected from the group consisting of glass, plastic,and metal.
 4. The method of claim 1, wherein the cathode layer is madeof a material selected from the group consisting of gold, silver,copper, and their alloys.
 5. The method of claim 1, wherein the carbonnanotube paste is prepared by mixing carbon nanotubes in a conductivepaste.
 6. The method of claim 5, wherein the conductive paste is silverpaste.
 7. The method of claim 5, wherein a mass percent of carbonnanotubes in the carbon nanotube paste is about 5%-15%.
 8. The method ofclaim 5, wherein a length of carbon nanotubes is about 5-15 microns. 9.The method of claim 1, wherein the carbon nanotube paste is coated onthe cathode layer by a screen-printing method.
 10. The method of claim1, wherein the carbon nanotube paste is calcined in air or in vacuum forapproximately 15 to 60 minutes.
 11. The method of claim 1, wherein thelaser beam irradiates a selective portion of a surface of the carbonnanotube composite layer.
 12. The method of claim 1, wherein the powerdensity of the laser beam is such that at least one protrusion is formedon the carbon nanotube composite layer of the field electron emissionsource, with at least one carbon nanotube projecting from the at leastone protrusion.
 13. The method of claim 1, wherein the power density ofthe laser beam is approximately 10⁴-10⁵ V/mm².
 14. The method of claim1, wherein the laser beam is moved along a predetermined route at a rateof around 800-1500 millimeters per second.
 15. A field electron emissionsource comprising: a substrate; a cathode layer deposited on thesubstrate; and a carbon nanotube composite layer coated on the cathodelayer, the carbon nanotube composite layer comprising a plurality ofcarbon nanotubes, at least one protrusion formed on the carbon nanotubecomposite layer with at least one carbon nanotube projecting from the atleast one protrusion.
 16. The field electron emission source of claim15, wherein the carbon nanotube composite layer further comprises aresultant paste.
 17. The field electron emission source of claim 15,wherein a weight ratio of the carbon nanotubes in the carbon nanotubecomposite layer is in the approximate range form 5% to 15%.